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Benthic fluxes of biogenic elements in the Baltic Sea: Influence of oxygen and macrofauna
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis investigates how benthic fluxes of phosphorus (P), nitrogen (N), and silicon (Si) change upon oxygenation of anoxic soft bottoms in the brackish, eutrophicated Baltic Sea. Direct measurements in situ by benthic landers demonstrated that fluxes of dissolved inorganic P (DIP) from anoxic bottom sediments in the Eastern Gotland Basin are higher than previously thought (Paper I). It is argued that the benthic DIP flux has a much larger influence on the DIP inventory in the Baltic proper than the external sources. Similarly, benthic fluxes of DIP and dissolved inorganic N (DIN) from anoxic sediment in the coastal Kanholmsfjärden Basin, Stockholm archipelago, were sufficiently high to renew the pools of these nutrients below the upper mixed layer in roughly one year (Paper II).

A natural inflow of oxygen rich water into the deep, and previously long-term anoxic part of Kanholmsfjärden Basin, increased the P content in the sediment by 65% and lowered DIP and dissolved silica (DSi) concentrations in the pore water. These changes, as well as the large increases in benthic effluxes of these solutes following de-oxygenation of the bottom water, suggest that they are influenced similarly by changing oxygen conditions.

Experimental results in papers III and IV show that common benthic macrofauna species in the Baltic Sea can stimulate benthic release of DIN and DSi, as well as dissolved organic and particulate bound nutrients. Thus, if benthic oxygen conditions would improve in the Baltic, initial effects on benthic–pelagic nutrient coupling will change due to animal colonisation of currently azoic soft bottoms.

A new box corer was designed (Paper V) which can be used to obtain highly needed virtually undisturbed samples from soft bottom sediments – if lowered slowly and straight into the bottom strata – as demonstrated by in situ videography and turbidimetry. The commonly used USNEL box corer caused severe biasing during sediment collection.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University , 2015. , 56 p.
Keyword [en]
Sediment biogeochemistry, Bioturbation, Benthic nutrient fluxes, Bottom water oxygenation, Box corer, Sediment sampling
National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
URN: urn:nbn:se:su:diva-114598ISBN: 978-91-7649-117-1 (print)OAI: oai:DiVA.org:su-114598DiVA: diva2:793649
Public defence
2015-04-16, Stora föreläsningssalen, Lilla Frescati, Lilla Frescativägen 5, Stockholm, 13:30 (English)
Opponent
Supervisors
Projects
Baltic oxygenation project
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 4: Manuscript. Paper 5: Manuscript.

Available from: 2015-03-25 Created: 2015-03-09 Last updated: 2015-03-25Bibliographically approved
List of papers
1. Phosphorus recycling in sediments of the central Baltic Sea
Open this publication in new window or tab >>Phosphorus recycling in sediments of the central Baltic Sea
Show others...
2013 (English)In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 10, no 6, 3901-3916 p.Article in journal (Refereed) Published
Abstract [en]

Benthic fluxes of phosphorus (P) were measured in situ in the Eastern Gotland Basin (EGB), central Baltic Sea, using benthic landers. A total of 40 flux measurements of dissolved inorganic P (DIP) on 13 stations at water depths ranging 30-210 m and under different oxygen regimes were carried out on three cruises during three consecutive years (2008-2010) in August-September. Our study is the first to report in situ DIP fluxes in the Baltic proper, and it provides the most comprehensive dataset of benthic fluxes of DIP and dissolved organic P (DOP) in the Baltic proper existing to date. DIP fluxes increased with increasing water depth and with decreasing bottom water oxygen concentration. Average DIP fluxes were calculated for oxic bottom water conditions (-0.003+/-0.040 mmol m(-2) d(-1)), hypoxic conditions (0.027+/-0.067 mmol m(-2) d(-1)) and anoxic conditions (0.376+/-0.214 mmol m(-2) d(-1)). The mean DIP flux at anoxic bottoms was higher than previous estimates based on ex situ measurements of pore water gradients. The DIP flux was positively correlated with the organic carbon inventory of sediment, and the benthic flux of dissolved inorganic carbon (DIC) at anoxic stations, but these variables were uncorrelated at oxic stations. The positive correlation between DIP and DIC fluxes suggests that the benthic DIP efflux from anoxic bottoms in the Baltic Proper is mainly controlled by rates of deposition and degradation of organic matter. The flux from anoxic sediment was very P rich in relation to both C and nitrogen (N). The average C : P ratio in fluxes at anoxic accumulation bottoms was 69+/-15, which is well below the Redfield C : P ratio of 106 : 1. At oxic stations, however, the C : P flux ratio was much higher than the Redfield ratio, consistent with well-known P retention mechanisms associated with iron and bacteria in oxidised sediment. Using a benthic mass balance approach, a burial efficiency estimate of 0.2-12% was calculated for the anoxic part of the EGB, which suggests that anoxic Baltic sediments are very efficient in recycling deposited P. Based on the measured fluxes and the average areal extent of anoxic bottoms during years 1999-2006, an internal DIP load of 152 k ton yr(-1) was calculated. This is almost 9 times higher than the average external total phosphorus (TP) supply to the Baltic proper during the same period. This comparison clearly highlights the dominance of internally regenerated P as a DIP source in the Baltic Sea.

National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-93203 (URN)10.5194/bg-10-3901-2013 (DOI)000321122700030 ()
Note

AuthorCount:5;

Available from: 2013-09-04 Created: 2013-09-04 Last updated: 2017-12-06Bibliographically approved
2. Recycling of biogenic elements in sediments of a stratified coastal Baltic Sea basin: Effects of bottom water oxygen oscillation
Open this publication in new window or tab >>Recycling of biogenic elements in sediments of a stratified coastal Baltic Sea basin: Effects of bottom water oxygen oscillation
(English)Manuscript (preprint) (Other academic)
National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-114592 (URN)
Available from: 2015-03-09 Created: 2015-03-09 Last updated: 2016-01-29Bibliographically approved
3. Recolonisation by macrobenthos mobilises organic phosphorus from reoxidised Baltic Sea sediments
Open this publication in new window or tab >>Recolonisation by macrobenthos mobilises organic phosphorus from reoxidised Baltic Sea sediments
2012 (English)In: Aquatic geochemistry, ISSN 1380-6165, E-ISSN 1573-1421, Vol. 18, no 6, 499-513 p.Article in journal (Refereed) Published
Abstract [en]

In recent decades, eutrophication has increased the extent of hypoxic and anoxic conditions in many coastal marine environments. In such conditions, the nutrient flux across the sediment–water interface is a key process controlling the biogeochemical dynamics, and thereby the level and character of biological production. In some areas, management attempts to drive the ecosystem towards phosphorus (P) limitation, which calls for reliable knowledge on the mechanisms controlling P-cycling. We report a well-controlled laboratory experiment on benthic fluxes of P, when shifting from a state of hypoxic and azoic sediments to oxic and zoic bottom conditions. Adding any of three types of macrobenthic fauna (mysid shrimp, pontoporeid amphipod and tellinid clam) to oxygenated aquarium sections resulted in benthic P fluxes that differed consistently from the azoic control sections. All species caused liberation of dissolved organically bound P (DOP) from the sediment, in contrast to the azoic systems. The shrimp and the amphipod also resuspended the sediment, which resulted in a release of P bound to particles (>0.45 μm). Dissolved inorganic phosphate (DIP) was released during hypoxic conditions, but was taken up after oxygenation, irrespective of the presence or absence of bottom fauna. In the presence of fauna, the uptake of DIP roughly equalled the release of DOP, suggesting that the benthic efflux of DOP following oxygenation and bottom fauna (re)colonisation might be considerable. This is an hitherto overlooked animal-controlled nutrient flux, which is missing from coastal marine P budgets.

Keyword
DOP, P retention, Bioturbation, Monoporeia affinis, Macoma balthica, Mysis mixta
National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-83090 (URN)10.1007/s10498-012-9172-5 (DOI)000312069200004 ()
Available from: 2012-12-04 Created: 2012-12-04 Last updated: 2017-12-07Bibliographically approved
4. Nutrient fluxes from reduced Baltic Sea sediments: Effects of oxygenation and macrobenthos
Open this publication in new window or tab >>Nutrient fluxes from reduced Baltic Sea sediments: Effects of oxygenation and macrobenthos
(English)Manuscript (preprint) (Other academic)
National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-114595 (URN)
Available from: 2015-03-09 Created: 2015-03-09 Last updated: 2016-01-29Bibliographically approved
5. Long over-due improvement of box corer sampling
Open this publication in new window or tab >>Long over-due improvement of box corer sampling
(English)Manuscript (preprint) (Other academic)
National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-114597 (URN)
Available from: 2015-03-09 Created: 2015-03-09 Last updated: 2016-01-29Bibliographically approved

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